i usually take my wheels into my LBS and see what they offer for the event. i have 16" wheels on my greenspeed, so i understand the limiting factor your going through. just taking them in to the LBS means they can measure and see what they have that's compatable n usually i get em to go a head and install the new tyre(s) cause new ones are a pain in the pitelly to put on! if the shop is slow when you go in i've notice you have more chance of getting tub/tyre installation thrown in free. if they are slammed they try and wrangle a 5 out of me for it... i can usually talk em out of it though.

good luck!
kat